In 1986, Bednorz and Muller initiated a series of discoveries of new high-Tc oxide superconductors when they found a Tc of about 30K in La.sub.1.85 Ba.sub.0.15 CuO.sub.4. The current highest-Tc compound is in the Tl-Ba-Ca-Cu-O system where the transition to zero resistance occurs at 125K. The earlier superconductors had generally been of Nb and its alloys and compounds. The newer category includes compounds discovered in 1988 in three different materials systems. It appears that the relevant properties of the Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O sets of compounds are similar to those of the rare-earth-Ba-Cu oxides, typified by YBa.sub.2 Cu.sub.3 O.sub.7 (YBCO), so YBCO will be used to represent the high-Tc oxide superconductors. Relatively little is known about the third new superconductor, Ba.sub.0.6 K.sub.0.4 BiO.sub.3, except that--unlike the higher-Tc compounds--it has a cubic structure and (presumably) isotropic properties.
The discovery of oxide superconductors with transition temperatures, Tc, greater than the boiling point of liquid nitrogen, 77K, has opened new possibilities for hybrid semiconductor/superconductor circuits. One of the possible applications is to use low-loss, dispersionless superconducting striplines as interconnects in semiconductor circuits. As shown in FIG. 1, the advantages of using superconducting interconnects in place of Al to reduce signal delay times are greatest for the longest interconnection paths. The present obstacle to developing such applications is the inability to grow high-Tc films, typified by YBa.sub.2 Cu.sub.3 O.sub.7 (YBCO), on silicon wafers without having a thick, insulating interface layer that prevents current transfer. The formation of a degraded substrate/film interface layer is a general problem in the growth of superconducting films. The origin is often reaction and interdiffusion with the substrate at temperatures needed to crystallize the superconducting film. At sufficiently low deposition temperatures, reaction with the substrate can be eliminated but crystalline disorder in the superconductor lowers Tc. Table 1 is a summary of the film thickness, d.sub.c, needed to obtain 75% of the Tc found in bulk superconductors compared to the superconducting coherence length, which is the minimum distance over which the Tc can vary from its full value to zero.
TABLE 1 ______________________________________ Minimum thickness, d.sub.c, required for non-epitaxial and epitaxial films to obtain &gt;75% of the Tc found in bulk samples. Required Fabri- d.sub.c (nm) d.sub.c cation (Non- (nm) Super- Temp. epitax- (Epi- .xi. (nm) conductor (.degree.C.) ial) taxial) (.parallel.Cu--O.) ______________________________________ Pb 20 3 -- 90 Nb 20-800 25 5 40 NbN 50-700 15 &lt;1 4 Nb3Sn 750-950 25 8 3 YBCO 600-900 400 00 3.1, 0.4 Bi--Sr--Ca--Cu--O 870 -- -- 3.1, 0.4 ______________________________________
Table 1 shows that d.sub.c &lt;coherence length for Pb and Nb and for NbN only in the case of epitaxial film growth. Therefore, if these films are grown on a metallic substrate, the substrate/film interface will be strongly superconducting. The contact formed in this manner can be expected to have zero resistance up to a critical current density, Jc, on the same order as bulk Pb or Nb. In contrast, YBCO films must be much thicker than a coherence length to obtain high Tc's. The standard approach to try to reduce d.sub.c for YBCO is to lower the film deposition temperature (or formation temperature for amorphous YBCO films that are post-annealed to become superconducting). Deposition temperatures have been lowered in a number of laboratories to about 600.degree. C. Some reports of successful growth at 400.degree. C. have been made, but there are questions about how accurately temperature was measured in those cases. The relatively low deposition temperatures have permitted growth directly on Si or on Si coated with a buffer layer, typically ZrO.sub.2. Although these efforts have reduced substrate/YBCO reaction, they have not resulted in sufficient crystalline order to lower d.sub.c below the values listed in Table 1. There is no prospect of a metallic buffer layer that will perform better than the insulating buffer layers that currently give the best results.
Since the problems related to large d.sub.c have not been solved, the only low-resistance contacts made to YBCO films have been made with Au contacting the top surface of the film. Techniques for forming these contacts at 20.degree. C. have been published. Other techniques have also resulted in low contact resistances for samples heated to &gt;300.degree. C.
It is not only degradation of the superconductor that must be considered in semiconductor/superconductor hybrids. Overall processing temperatures must be kept low enough to prevent diffusion profiles from changing in semiconductor devices. Deposition temperatures for YBCO of about 600.degree. C. are low enough to be compatible with Si circuits, but are too high or, at best, marginal for GaAs, which starts to decompose at 580.degree. C.
Contacts between most semiconductors and metals are rectifying. For Si, the standard metal for ohmic contacts is Al. While there are other suitable metals, the choice of metals for contacts is severely constrained. Unfortunately, Al is a particularly reactive metal in contact with YBCO.